1. Field of the Invention
The present invention relates to an image display apparatus that displays various images and a method of driving the image display apparatus.
2. Related Background Art
(1) Generally, display images have different aspect ratios (ratios between horizontal sizes and vertical sizes), depending on the types of image sources. The screen sizes (length-to-width ratios of screens) of image display apparatuses have conventionally been set so as to match the aspect ratios of images to be displayed. As shown in FIGS. 1A and 1B, however, there may be cases where the aspect ratios (x1:y1 and x1:y3) of screens do not match the aspect ratios (x2:y1 and x1:y2) of images. This problem is described in more detail below.
It is currently required that image display apparatuses display various types of images, such as television images and Internet images, that have different aspect ratios. FIG. 7A shows an example of an Internet image that is displayed on a personal computer screen and has an aspect ratio of x2:y1=4:3, while FIG. 7B shows an example of a television image that is displayed on the screen of a wide television set and has an aspect ratio of x1:y2=16:9.
It has conventionally been sufficient that image display apparatuses of television sets only display television images and image display apparatuses of personal computers only display specific images such as Internet images. That is, the aspect ratios of images to be displayed by image display apparatuses have been predetermined and the screen sizes (aspect ratios of screens) of the image display apparatuses have been set to match the aspect ratios of images to be displayed.
Recent advancements in the multimedia field, however, make image display apparatuses not only to display specific images but increases the opportunities for them to display images in various image signal formats. For instance, television sets (image display apparatuses) capable of displaying Internet images and conversely personal computers (image display apparatuses) capable of displaying television images are now on the market. These image display apparatuses are designed not only to display images having a fixed aspect ratio but to display images having various aspect ratios.
Also, there appear television images having various aspect ratios. That is, images broadcasted by terrestrial analog broadcast services have an aspect ratio of 4:3, while images broadcasted by satellite broadcast services or digital broadcast services have an aspect ratio of 16:9. This raises the possibility that even if image display apparatuses display only television images and do not display Internet images, images displayed by them vary in the aspect ratio.
As shown in FIGS. 1A and 1B, if images are displayed by image display apparatuses whose screen sizes do not match the aspect ratios of the images, the screen areas of the image display apparatuses are divided into two types of portions: portions B1 (hereinafter referred to as “effective image areas B1”) where various images are displayed, and portions B2 (hereinafter referred to as “non-effective image areas B2”) where no image is displayed and masks are applied. Note that FIG. 1A shows a state where an Internet image (aspect ratio=4:3) is displayed on an image display apparatus whose screen aspect ratio is 16:9, while FIG. 1B shows a state where an image (aspect ratio=16:9) is displayed on an image display apparatus whose screen aspect ratio is 4:3. In either case of these image display apparatuses, black masks are displayed in the non-effective image areas B2.
(2) Image displays have conventionally been performed by sequentially scanning pixels that are capable of performing multi-level display and are arranged within display screens, although there appear on the market display apparatuses adopting a different display method where image display (multi-level gradation display) is performed by performing time divisional display of each display value subjected to a pulse width modulation (PWM) using pixels for binary display.
FIG. 2 shows an example construction of an image display apparatus (projection-type display apparatus using a mono-plate scheme) that performs the time divisional display. Here, the term “mono-plate scheme” means a method of displaying images in each color (such as, red (R), green (G), and blue (B)) using a single spatial modulation element (image display element). This method simplifies optical systems and electric circuit systems and therefore is suitable for realizing a low-cost and lightweight display unit.
An image display apparatus 1 in FIG. 2 includes a binary-display-type image display element 2, such as an MEMS (micro-electromechanical systems) spatial modulation element. The image display element 2 is also of a reflection type and reflects light. On the side, toward which the image display element 2 reflects light, are arranged a screen 4, on which images are to be projected, and an optical system 5 for projecting reflection light (light that has been spatially modulated by the image display element 2 and includes display information) onto the screen 4. Note that reference numeral 50 represents a lens.
A lighting device 3 is provided with a metal halide lamp 30 that emits white light using power supplied by a ballast power source 31. A disc-like rotary color filter 32 is disposed between the lamp 30 and the image display element 2 so as to be freely rotated and the color filter 32 is structured so as to be rotated and driven by a filter driving unit 33. Here, as shown in FIG. 8, the color filter 32 is divided into three color regions 32R, 32G, and 32B. Light in three colors (red, green, and blue) is sequentially irradiated onto the image display element 2 according to the rotation of the color filter 32.
Note that reference numeral 34 indicates a lens disposed between the color filter 32 and the lamp 30, and numeral 35 indicates a lens disposed between the color filter 32 and the image display element 2.
Also, reference numeral 7 represents an input unit for inputting image signals. Further, reference numeral 8 denotes a signal processing unit that processes the inputted image signals by adjusting image quality (such as brightness, color characteristics, and gamma characteristics) of the inputted image signals and converting the adjusted image signals into PWM-modulated time divisional signals that are appropriate for the driving method of the display element. The signal processing unit 8 also generates a driving pulse for the display element, a control signal for a motor, and the like. Reference numeral 8a indicates a data bus that transmits the time divisional signals to the display element, and numeral 8b indicates a control line that transmits the driving pulse to the display element.
According to these signals from the signal processing unit 8, the image display element 2 sequentially displays images in synchronization with light irradiation. In this manner, images in different colors are sequentially displayed on the screen 4, on which these images are mixed visually and are recognized as full-color images by viewers.
The construction of the signal processing unit 8 stated above is described in more detail below with reference to FIG. 9. Here, FIG. 9 is a block diagram showing the detailed construction of the signal processing unit 8.
In this drawing, an input unit 7 for inputting various image signals includes an input terminal 71 for inputting an image signal, an input terminal 72 for inputting a horizontal synchronizing signal (IHD) among the input signals, an input terminal 73 for inputting a vertical synchronizing signal (IVD) among the input signals, and an input terminal 74 for inputting a clock signal (ICLK) among the input signals.
In this drawing, reference numerals 711, 712, 713, and 714 each represent a data bus for transmitting these image signals. Reference numeral 721 indicates a signal line for transmitting the horizontal synchronizing signal (IHD) among the input signals, numeral 731 indicates a signal line for transmitting the vertical synchronizing signal (IVD) among the input signals, and numeral 741 indicates a signal line for transmitting the clock signal (ICLK) among the input signals.
Reference numeral 80 denotes an image input unit. In more detail, the image input unit 80 is an image signal receiving unit. For instance, the image input unit 80 includes a decoder that receives a signal based on a TMDS scheme and decodes the received signal into 24-bit data (three pieces of 8-bit data corresponding to respective colors (R, G, and B)). Here, the TMDS scheme is an image transmission scheme adopted by, for instance, a DVI (Digital Visual Interface) specification published by a standardizing group “DDWG (Digital Display Working Group)”. Alternatively, the image input unit 80 includes a decoder that receives a compression signal in an MPEG format via IEEE 1394 and decodes the received compression signal into 24-bit data (three pieces of 8-bit data corresponding to respective colors (R, G, and B)).
Reference numeral 81 represents a format conversion unit that performs resolution conversion, image refresh frequency conversion, non-interlace processing, color matrix conversion, and the like. Here, the resolution conversion means magnification conversion and interpolation processing that are appropriately performed for an image signal whose resolution does not match the number of display pixels of the image display unit. Also, reference numeral 82 represents a memory unit that provides an image storage area used by the format conversion unit to perform the image processing. Reference numeral 82a indicates a control line group of the memory unit, and numeral 82b indicates a data line group for transferring data between the memory unit and the format conversion unit. Reference numeral 83 denotes a crystal oscillator. According to the clock signal (OCLK) generated by the crystal oscillator, the format conversion unit 81 generates a horizontal synchronizing signal (OHD) and a vertical synchronizing signal (OVD), which are used to establish synchronization after the format conversion processing, under the control by a microcomputer unit (not shown). Reference numeral 811 indicates a signal line for transmitting the horizontal synchronizing signal (OHD), numeral 812 indicates a signal line for transmitting the vertical synchronizing signal (OVD), and numeral 813 indicates a signal line for transmitting the clock signal (OCLK) generated by the crystal oscillator.
Reference numeral 84 represents an image quality adjusting unit that receives the image signal subjected to the format conversion and adjusts image quality, such as brightness, color characteristics, and gamma characteristics, of images to be displayed on the display unit, according to the control by the microcomputer (not shown).
Reference numeral 85 indicates a PWM conversion unit for converting an ordinary image signal for sequential scanning into a time divisional display signal by performing the pulse width modulation (PWM), numeral 86 indicates a time divisional sequence storage unit for storing time divisional drive sequence data describing the display order and display time period of the PWM-modulated data, numeral 87 indicates a PWM driving timing generating unit for generating, according to the time divisional drive sequence, driving timing used by the PWM conversion unit 85 and the spatial modulation element (image display element) that is an image display unit. Reference numeral 861 denotes a transmission line for transmitting the drive sequence data from the time divisional drive sequence storage unit 86 to the PWM drive timing generating unit 87, and numeral 871 indicates a control line group for transmitting a driving pulse generated by the PWM driving timing generating unit 87 and other signals. Also, reference numeral 872 represents an output terminal via which control signals, such as the driving pulse, are outputted to the image display element 2, numeral 851 a data bus for transmitting the image data converted by the PWM conversion unit 85, and numeral 852 indicates an output terminal via which the image data is outputted to the image display element 2.
The PWM drive timing generating unit 87 generates the control signal for the PWM conversion unit 85 and the driving pulse for the display element according to the sequence data in the time divisional sequence storage unit 86. That is, the image inputted into the signal processing unit is subjected to appropriate format conversion and image quality adjustment and then is converted into the time divisional drive signal by the PWM conversion unit 85. The PWM conversion unit 85 and the display element are driven in synchronization with each other.
FIG. 10 shows an example of the display data sequence that has been PWM-modulated by the PWM conversion unit 85. In this drawing, the horizontal axis represents time and reference numeral 201 denotes a start pulse designating the start of image display in each color (R, G, and B) within one field. Reference symbol FR indicates a time period during which red display is performed, reference symbol FG indicates a time period during which green display is performed, and reference symbol FB indicates a time period during which blue display is performed. In this specification, a time period composed of one FR period, one FG period, and one FB period is referred to as one field period.
Also, reference symbols DR1–DR6 represent display data in red that has been PWM-modulated. Here, for ease of explanation, the display data is expressed as 6-bit signal, with reference symbol DR1 representing the first-bit signal, reference symbol DR2 the second-bit signal, reference symbol DR3 the third-bit signal, reference symbol DR4 the fourth-bit signal, reference symbol DR5 the fifth-bit signal, and reference symbol DR6 the sixth-bit signal. The pulse length of each bit signal is twice as long as that of the next lower bit signal. For instance, the length of the second-bit signal DR2 is twice as long as that of the first-bit signal DR1 and the length of the third-bit signal DR3 is twice as long as that of the second-bit signal DR2. According to the image data inputted by the PWM conversion unit 85, each bit is selected so that the pulse width matches the gradation value of the image data. In this manner, a time series ON/OFF signal subjected to the pulse width modulation is obtained. According to this ON/OFF signal, each pixel of the image display element 2 is placed in one of the binary states. By performing light reflection in one of the binary states, an image in red is displayed within one field period according to the integral in the FR period.
Reference symbols DG1–DG6 represent display data in green that has been PWM-modulated, and reference symbols DB1–DB6 represent display data in blue that has been PWM-modulated. In either case of green display data and blue display data, the pulse length of each bit signal is twice as long as that of the next lower bit signal. According to the image data inputted by the PWM conversion unit 85, a signal having a pulse width corresponding to the gradation value of the image data is generated. The image display element 2 is driven and light reflection is controlled according to the signal subjected to the pulse width modulation. Images in green and blue are displayed within one field period according to the integral value in the FG period and the integral value in the FB period.
In this manner, a full-color image in one field is displayed according to the integral in each color period in one field.
As described above, image (gradation) display in the effective image areas B1 is performed by placing each pixel of the image display element 2 in one of binary display states according to a pulse train that has been PWM-modulated based on the gradation value of image data in each color. (Here, in this specification, a state where light is reflected is referred to as an “ON state” and a state where light is not reflected is referred to as an “OFF state”.) That is, image display is performed according to the integral of one of binary display states. Consequently, as distinct from an analog gradation TFT liquid crystal, the state of each pixel of such a binary-type image display element is switched between the ON state and OFF state in one field period even during still image display.
On the other hand, no image is basically displayed in the non-effective image areas B2, so that each pixel in these areas B2 of the binary image display element 2 is continuously placed in the OFF state and dark display is performed. In this example where display data is expressed as 6-bit signal, the dark display corresponds to a situation where each of RGB (red, green, and blue) has shade 0 among 64 (0–63) shades of gray scale.
It should be noted here that an example measure against hinge storage (to be described later) is disclosed in Japanese Patent Application Laid-open No. 08-195963.
Also, Japanese Patent Application Laid-open No. 09-322101 discloses a measure against image burn-in (to be described later). This patent application discloses a measure against image burn-in on a CRT caused by still image display. With this technique, the input current into the fluorescent surface of the CRT is maintained basically constant during both of display time periods and non-display time periods.
Another conventional technique of preventing image burn-in is disclosed in Japanese Patent Application Laid-open No. 5-153529. This patent application discloses a technique of achieving a liquid crystal display panel, which is easy to view, and of preventing image burn-in on the display panel. In particular, with this technique, white display is performed in side panel areas of the liquid crystal display panel for a predetermined time period before a display operation is stopped.
Japanese Patent Application Laid-open No. 5-122633 discloses still another conventional technique of reducing a brightness unevenness in non-image areas occurring when an image whose aspect ratio is 4:3 is displayed on a wide aspect television set. With this conventional technique, if non-image areas are generated on the screen of a cathode ray tube due to the display of a 4:3 image, light emission is performed in the non-image areas for a time period before the system is turned off, with the time period being determined according to the display time period of the image signal whose aspect ratio is 4:3.